Results of Phase I studies with new recombinant strains of attenuated Salmonella typhi used as live oral vaccines suggest that these well- tolerated vaccines can elicit potent immune responses. Such attenuated S. typhi are also attractive candidates to serve as "live vector vaccines" to express foreign antigens of other pathogens and delivery them to the human immune system. The development of improved typhoid vaccines has been hampered by a considerable lack of information regarding human cellular mucosal immune (CMI) responses to S. typhi infection, likely to be a key defense mechanism. The broad objective of this project is to characterize, at the mucosal level, the CMI that follow oral immunization with the new generation of attenuated S. typhi strains and to identify the responses that best correlate with protection. For this purpose, we will use cell populations isolated from mucosal biopsies of immunized volunteers. It was demonstrated that CTL can also play an important role in the defense against intracellular bacterial pathogens. We will test the hypothesis that CTL responses play a crucial role in limiting the progression of typhoid infection by the destroying host cells harboring the intracellular pathogen S. typhi. We further hypothesize that another key component in the induction of protective immunity is the triggering of T(H)1-type responses, with production of IFN-gamma and other key cytokines that enhance the activity of macrophages and other effector cells. Specifically, we will: 1. Test the hypothesis that protection against S. typhi depends, at least in part, on CTL responses at effector sites in the human gut mucosa. This will be tested in volunteers vaccinated with attenuated strains of S. typhi or challenged with wild-type S. Typhi, 2. Test the hypothesis that specific T(H)1 responses occur during S. typhi infection and correlate with protective immunity and generation of CTL responses and 3. Test the hypothesis that antigen-specific clonal T cell populations can be isolated from the intestinal mucosa that retain the same functional characteristics of populations in the mucosa following S. typhi infection. These studies will provide invaluable information about the determinants of protective CMI responses to S. typhi at the mucosal site and will facilitate and accelerate the design of new live vector vaccines.